Modulating Control Valves and Lift? (a puzzle for the experts)

George Denecke

We have the opportunity to correct a system that is being put in a school out here the wrong way. I wonder if anyone has ideas on the best way to fix it before it gets put in operation:

On 80 units we currently have 6 Psi steam going through modulating control valves and into a fan coil unit and an F&T Trap. It then supposedly has to be lifted 18" to get back into a vacuum condensate return. The FCU's have vacuum breakers on them as well.

I do not believe that this system will work as it is currently installed. To fix it I think we have to go with thermostatic steam traps with a check valve in front of them but I'm not sure if that is enough. Is there still going to be enough water hammer when the control valve opens to destroy the trap/coil in short order? Do we have to remove the control valves and deal with a system that is too hot in the spring and fall (and wastes energy)?. Any other ideas to fix it?

I think the ideal situation would be to repipe it as a gravity return, but this seems like too costly a solution on a job that started out as a tight bid and the piping is almost complete.

George Denecke

Modulating Control Valves and Lift? (a puzzle for the experts)

We have the opportunity to correct a system that is being put in a school out here the wrong way. I wonder if anyone has ideas on the best way to fix it before it gets put in operation:

On 80 units we currently have 6 Psi steam going through modulating control valves and into a fan coil unit and an F&T Trap. It then supposedly has to be lifted 18" to get back into a vacuum condensate return. The FCU's have vacuum breakers on them as well.

The problems that are most obvious to me are that the condensate will get hung up in the coil when the control valve shuts, and when it is open the trap will get caved in by water hammer. If it does get running the F&T trap will pass the condensate at a temperature too high and it will flash back into steam in the vacuum return.

I do not believe that this system will work as it is currently installed. To fix it I think we have to go with thermostatic steam traps with a check valve in front of them but I'm not sure if that is enough. Is there still going to be enough water hammer when the control valve opens to destroy the trap/coil in short order? Do we have to remove the control valves and deal with a system that is too hot in the spring and fall (and wastes energy)?. Any other ideas to fix it?

I think the ideal situation would be to repipe it as a gravity return, but this seems like too costly a solution on a job that started out as a tight bid and the piping is almost complete.

Ken_8

I too have seen vacuum added

to existing condensate receivers and have found the need more a game of an over zealous engineer than true need. We take care of lots of schools and found none that had vacuum systems required them.

We also found these were old coal systems converted in the 50's to oil and the 5 PSI settings were totally arbitrary. We dropped the pressures to full fire at 1 PSI and full mod to 2 PSI as the low fire limit. 3 PSI was the operating pressuretrol setting and the MR high limit set to 5 PSI. Gravity is best if at all possible. Traps of course are key - and mostly overlooked by most.

If pitch looks decent, try turning off the vacuum p8umps and see what happens. Better to have perimeter receivers than vacuum that's not needed.

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bob_25

lift

george, in my experance you CAN'T lift condensate with a modulating valve, period. I should re-phrase that you can't lift condensate and accuratly control temperature with a modulating valve. The coil will flood at low demand. bob